There is a growing demand for transportation fuels made from renewable feedstocks. These renewable fuels displace fossil fuels resulting in a reduction of greenhouse gas emissions, along with other benefits (1-3). Biofuels include fuel ethanol. Fuel ethanol is roduced from biomass by converting starch or cellulose to sugars, fermenting the sugars to ethanol, and then distilling and dehydrating the ethanol to create a high-octane fuel that can substitute in whole or in part for gasoline.
In North America, the feedstock for the production of fuel ethanol is primarily corn, while in Brazil sugar cane is used. There are disadvantages to using potential food or feed plants to produce fuel. Moreover, the availability of such feedstocks is limited by the overall available area of suitable agricultural land. Therefore, efforts are being made to generate ethanol from non-food sources, such as cellulose, and from crops that do not require prime agricultural land, for example miscanthus. Cellulose is one of the most abundant organic materials on earth. It is present in many forms of biomass, including agricultural residues like corn stover and corncobs, woody residues and other plant materials. Cellulose is a polymer of glucose, as is starch. However, the isolation of reactive cellulose from lignocellulosic biomass and hydrolysis to C6 sugar monomers has its challenges. One non-food source of C6 sugars is lignocellulosic biomass.
Lignocellulosic biomass may be classified into four main categories: (1) wood residues (sawdust, bark or other), (2) municipal paper waste, (3) agricultural residues (including corn stover, corncobs and sugarcane bagasse), and (4) dedicated energy crops (which are mostly composed of fast growing tall, woody grasses such as switchgrass and miscanthus).
Lignocellulosic biomass is composed of three primary polymers that make up plant cell walls: Cellulose, hemicellulose, and lignin. Cellulose fibres, which contain only anhydrous glucose (C6 sugar), are locked into a rigid structure of hemicellulose and lignin. Lignin and hemicelluloses form chemically linked complexes that bind water soluble hemicelluloses into a three dimensional array, cemented together by lignin. Lignin covers the cellulose microfibrils and protects them from enzymatic and chemical degradation. These polymers provide plant cell walls with strength and resistance to degradation, which makes lignocellulosic biomass a challenge to use as substrate for biofuel production.
Hemicelluloses are polysaccharides and include xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan, which all contain many different C5 or C6 sugar monomers. For instance, besides glucose, sugar monomers in hemicellulose can include xylose, mannose, galactose, rhamnose, and arabinose. Hemicelluloses contain most of the D-pentose sugars, and occasionally small amounts of L-sugars as well. Xylose is always the sugar monomer present in the largest amount, which is why hemicellulose content is often expressed in terms of xylose equivalent content, as will be discussed further below. Xylose is a monosaccharide of the aldopentose type, which means it contains five carbon atoms (C5 sugar) and includes an aldehyde functional group. Cellulose is crystalline, strong and resistant to hydrolysis, while hemicellulose has a random, amorphous structure with little strength and is easily hydrolyzed by dilute acid or base, or by hemicellulase enzymes.
There are two main approaches to the production of fuel ethanol from biomass: thermochemical and biochemical. Thermochemical processes convert the biomass to a reactive gas called syngas. Syngas is converted at high temperature and pressure to ethanol by a series of catalyzed processes. Biochemical processes use biocatalysts called enzymes to convert the cellulose content to sugars (C5 and C6), which are then fermented to ethanol and other fuels such as butanol. The biochemical processes generally exploit the different susceptibility to hydrolysis of hemicellulose and cellulose, by hydrolyzing the hemicellulose and cellulose in different steps.
Biochemical conversion of lignocellulosic biomass to ethanol in general involves five basic steps (1) Preparation—the target biomass is cleaned and adjusted for size and moisture content; (2) Pretreatment—exposure of the raw biomass to elevated pressure and temperature for a specified duration; with or without catalyzing additives to hydrolyze the hemicellulose separately from the cellulose; (3) Cellulose hydrolysis—conversion of the cellulose in the pretreated biomass to simple C6 sugars using special enzyme preparations to hydrolyze the pretreated plant cell-wall polysaccharides; (4) Fermentation, mediated by bacteria or yeast, to convert these sugars to fuel such as ethanol; and (5) Distillation and Dehydration of the ethanol/fuel.
Certain pretreatment methods employ chemical additives, such as acids, to catalyze the hydrolysis of hemicellulose and/or alkalis to remove lignin. These additives as well as many of the breakdown products they generate during the pretreatment process, such as lignin and some soluble lignin derivatives, are either toxic to yeast, or inhibit hydrolysis, or both. Furthermore, all forms of lignocellulosic biomass have some level of sterols, fatty acids, ethers and other extractives that can also be inhibitory.
One approach to address the inhibitory effect of these substances is the use of harsher pre-treatment conditions, which can for example be tailored to effectively hydrolyze and degrade the hemicellulose to such an extent that very little xylose and xylo-oligosaccharides remain to interfere with the cellulose enzymes. However this approach creates another significant disadvantage in that it causes significant cellulose degradation, which then reduces glucose yield and ultimately the ethanol yield, often creating a commercially significant reduction of the overall ethanol process efficiency, even in the virtual absence of inhibitory compounds.
In another approach xylanases are used to completely hydrolyze the xylan oligomers to xylose and lessen the inhibitory effect of these oligomers. However, although this approach is somewhat effective, it produces high levels of xylose which is itself an inhibitor. Moreover, the other inhibitory compounds generated in the pretreatment step from decomposition of the hemicellulose are still present. Thus, although the overall yield is better, in the end this approach is not commercially viable due to the added cost for the xylanases and the cost of still required elevated cellulase levels due to the other inhibitory substances.
All pretreatment processes, generally result in significant breakdown of the biomass, in particular the hemicellulose component, which leads to the generation of various C5 sugars and other hemicellulose breakdown products. Hemicellulose decomposition products such as formic acid, furfural and hydroxyl methyl furfural etc. are produced during pretreatment which carry through to and inhibit the hydrolysis and fermentation processes. Thus, these hemicellulose decomposition products reduce the effectiveness of the cellulose hydrolyzing enzymes, thereby requiring the use of increased levels of added enzyme, the cost of which is an important factor in providing a cost effective commercial process.
The breakdown products inhibitory to the downstream cellulose and/or sugar fermentation processes are usually separated from the biomass prior to cellulose hydrolysis, to minimize any potentially inhibitory effects on ethanol yield. However, although the overall ethanol yield could be significantly improved if the C5 sugars originating from the hemicellulose could also be used in the sugar fermentation step, separating the C5 sugars from the removed inhibitory hydrocellulose breakdown product stream is cost intensive and uneconomical. Thus, an efficient and economical process is desired which increases the yield of C5 and C6 sugars in a conventional lignocellulosic biofuel production process.